Heart assist device with expandable impeller pump

ABSTRACT

An impeller includes a hub and a blade supported by the hub. The impeller has a stored configuration in which the blade is compressed so that its distal end moves towards the hub, and a deployed configuration in which the blade extends away from the hub. The impeller may be part of a pump for pumping fluids, such as blood, and may include a cannula having a proximal portion with a fixed diameter, and a distal portion with an expandable diameter. The impeller may reside in the expandable portion of the cannula. The cannula may have a compressed diameter which allows it to be inserted percutaneously into a patient. Once at a desired location, the expandable portion of the cannula may be expanded and the impeller expanded to the deployed configuration. A flexible drive shaft may extend through the cannula for rotationally driving the impeller within the patient.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.16/181,963, filed Nov. 6, 2018, which is a continuation of U.S.application Ser. No. 15/633,189, filed Jun. 26, 2017, now U.S. Pat. No.10,149,932, which is a continuation of U.S. application Ser. No.15/176,620, filed Jun. 8, 2016, now U.S. Pat. No. 9,717,833, which is acontinuation of U.S. application Ser. No. 13/618,071, filed Sep. 14,2012, now U.S. Pat. No. 9,364,593, which is a continuation of U.S.application Ser. No. 12/945,594, filed on Nov. 12, 2010, now U.S. Pat.No. 9,364,592, which is a continuation of U.S. application Ser. No.11/728,051, filed on Mar. 23, 2007, now U.S. Pat. No. 7,841,976, whichclaims the benefit of the filing dates of U.S. Provisional ApplicationNo. 60/785,299, filed on Mar. 23, 2006 and 60/785,531, filed on Mar. 23,2006, the disclosures of which are hereby incorporated by referenceherein in their entirety for all purposes. Further, the entiredisclosures of U.S. Provisional Application No. 60/610,938, filed onSep. 17, 2004, and U.S. patent application Ser. No. 11/227,277, filed onSep. 15, 2005, are hereby incorporated by reference herein in theirentirety for all purposes.

FIELD OF THE INVENTION

The present invention relates to fluid pumping impellers, moreparticularly to expandable fluid pumping impellers. Still moreparticularly, the present invention relates to blood pumps such as leftor right ventricular assist devices with an expandable impeller fortreatment of heart disease.

BACKGROUND OF THE INVENTION

Heart disease is a major problem in society, and claims many lives peryear. After a heart attack, only a small number of patients can betreated successfully and non-invasively using medicines, such aspharmaceuticals. However, with sufficient mechanical assistance to theheart function, a majority of patients may recover from a heart attack,including even those with cardiogenic shock.

In a conventional approach, a blood pump having a fixed cross-section issurgically inserted within the left ventricle of the heart and theaortic arch to assist the heart in its function. Surgical placement isrequired, since it is presently impractical or impossible to insert apump of the size needed for sustaining adequate blood flowpercutaneously. The object of the surgically inserted pump is to reducethe load on the heart muscle for a period of time, which may be as longas a week, allowing the affected heart muscle to recover while healingin a resting mode.

Surgical insertion, however, can cause additional serious stresses inheart failure cases. Percutaneous insertion of a left ventricular assistdevice (“LVAD”) therefore is desired. However, the conventional fixedcross-sectional diameter of such an LVAD cannot fit through the femoralartery of the leg in which it must travel to be positioned into the leftventricle. The maximum diameter of such a fixed diameter LVAD would haveto be limited to approximately four millimeters for practicalpercutaneous insertion. This would limit the maximum pumped blood flowrate to approximately two liters per minute, approximately one-half thedesired sustaining blood flow value for many cases. While the pumpingrate can be increased by increasing the diameter of the device,particularly the diameter of the impeller, the size of the femoralartery is a limiting factor for percutaneous insertion. Hence, there isan urgent need for a pumping device that can be implanted throughpercutaneous insertion and yet provide the sustaining blood flow ratesthat conventional surgically implanted pumps provide.

SUMMARY OF THE INVENTION

The present invention may be used as an LVAD, a right ventricular assistdevice (“RVAD”) or in other situations that may benefit from a bloodpump that is expandable in situ after being inserted into the body of apatient. The blood pump has an impeller design that allows compressionand expansion of the impeller at the discretion of the operator. Thiscompression/expansion feature allows for increased blood flow throughthe blood pump due to an increase, by expansion, of the impeller size,thereby producing a blood flow capable of sustaining human life withoutthe need for significant contribution by the heart muscle. The bloodflow provided is typically at least 4 liters per minute of blood, theflow that is usually sufficient to sustain life.

The difference in using the blood pump as an LVAD as opposed to an RVADis the location of the pump in the patient and the flow direction ofblood through the pump.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is an end view of an embodiment of an impeller having three rowsof blades (blade rows);

FIG. 1B is a perspective view of the impeller of FIG. 1A;

FIG. 2A is a highly schematic side elevational view of an embodiment ofan impeller according to the present invention in its deployedconfiguration;

FIG. 2B is a highly schematic side elevational view of the impeller ofFIG. 2A in its stored configuration;

FIG. 3 is a highly schematic side elevational view schematicallyillustrating the deployment of the impeller of FIG. 2A;

FIG. 4A is an enlarged perspective view of a portion of a blade havingan embodiment of a winglet;

FIG. 4B is a cross-sectional view of the blade of FIG. 4A within aportion of a vessel;

FIGS. 5A-5C are cross-sectional views of blades having exemplary wingletconfigurations;

FIG. 5D shows cross-sectional views of exemplary winglet edge geometries

FIGS. 6A-6D are end views of an impeller blade, further illustratingpossible winglet configurations;

FIG. 7A is an enlarged perspective view of a portion of an impelleraccording to the present invention having an indentation in the hubsurrounding the proximate end of the blade;

FIG. 7B is a cross-sectional view of the portion of the impeller shownin FIG. 7A;

FIG. 8 is a stress-strain graph for a polyurethane material used to forman impeller blade;

FIG. 9A is a perspective view of an embodiment of an impeller in astored configuration within a storage housing;

FIG. 9B is a perspective view of the impeller of FIG. 9A after emergencefrom the storage housing;

FIG. 10 is a perspective view superimposing deployed and operationalconfigurations of an embodiment of an impeller;

FIG. 11 is a graph showing normalized average fluid shearing stresses asa function of normalized volume flow rates;

FIG. 12 is a graph showing normalized average fluid shearing stresses asa function of tip gap sizes;

FIG. 13 is a side view of another embodiment of an impeller according tothe present invention;

FIG. 14 is a perspective view of a blood pump according to the presentinvention;

FIG. 15A is a side elevational view of the impeller portion of the bloodpump of FIG. 14;

FIG. 15B is a side elevational view of a cannula in which the impellerof the blood pump of FIG. 14 operates;

FIG. 15C is a partial longitudinal cross-sectional view of the retainersheath for use with the blood pump of FIG. 14;

FIG. 16 shows highly schematic side views of different mesh designs; and

FIG. 17 is a perspective view of the expandable portion of the cannulashown in FIG. 14 in the deployed state;

FIG. 18 is an enlarged perspective view of the discharge or proximal endof the expanded cannula having a hexagonal mesh;

FIG. 19 is an enlarged perspective view of the inlet or distal end ofthe expanded cannula showing a guide wire having a distal tip;

FIG. 20 is an enlarged longitudinal cross-sectional view of an alternateembodiment of the distal end of the cannula forming a dilator;

FIGS. 21A and 21B are side elevational views of the expandable portionof the cannula in stored and deployed configurations, respectively;

FIGS. 22A and 22B are longitudinal highly schematic views of the bloodpump of the present invention in the deployed and stored configurations,respectively, showing system components;

FIG. 23A is a longitudinal cross-sectional view of the blood pump ofFIG. 14 in its deployed configuration;

FIG. 23B is a longitudinal cross-sectional view of the blood pump ofFIG. 14 in its retracted position;

FIG. 23C is a side elevational view of a cannula in which the impellerof the blood pump of FIG. 14 operates;

FIG. 24 is a longitudinal cross-sectional view of an alternateembodiment of a blood pump in its deployed configuration;

FIG. 25 is a side elevational view in partial cross-section showing apre-parked sheath embodiment of the present invention; and

FIG. 26 is a highly schematic view showing the blood pump deployment ina patient.

DETAILED DESCRIPTION

An impeller according to embodiments of the present invention includes ahub, and at least one blade supported by the hub. The impeller may havea deployed configuration in which the blade extends away from the hub,and a stored configuration in which the impeller is radially compressed,for example by folding the blade towards the hub.

In some embodiments, the outer edge of a blade may have a winglet. Theimpeller also may have a trench or indentation proximate to a blade rootto facilitate folding of the blade and/or to reduce shear stresses inthe fluid flow induced by rotation of the impeller.

Some embodiments of the present invention include impellers that do notradially compress, but retain a generally constant configuration.Impellers according to the present invention may be used in variousapplications, including improved blood pumps.

Blade Rows

Impellers according to embodiments of the present invention may includea plurality of blades which may be arranged in one or more blade rowspositioned along the impeller hub. Figures IA and IB illustrate end andside views, respectively, of an impeller 100. The impeller includes ahub 10, and a plurality of blades 12 arranged in three blade rows. Thefirst blade row includes blades 102 and 104, the second blade rowincludes blades 106 and 108, and the third blade row includes blades 110and 112.

The provision of a plurality of blade rows facilitates the folding ofthe blades into a stored configuration as compared to the difficulty offolding a single helical blade extending a similar distance along thehub. Each blade row may include the same number of blades, for example,one to three blades. Alternatively, the number of blades in each bladerow may differ. For embodiments in which there are more than two bladerows, the number of blades in at least one blade row may differ from thenumber of blades in other blade rows. The provision of a plurality ofblade rows facilitates the achievement of larger values of fluid head orpressure rise than a single blade row, while allowing the impeller to beradially compressed while allowing into a stored configuration.

One approach to impeller design provides an impeller having a longhelical blade exhibiting a significant degree of wrap around the centralhub. However, the three-dimensional shape of long helical blades limitsthe degree to which they can be folded without breaking or permanentlydeforming. By dividing a single helical blade into a plurality (two,three or possibly more) of individual blades, arranged in blade rows,the blades in each, row exhibit less wrap around the hub. Therefore, theindividual blades may have an essentially two-dimensional shape whichallows easier deformation during the storage process. The combination oftwo or more blade rows can produce the same flow and pressure as asingle helical blade of similar axial extent. For example, individualblades may have a height-to-chord length ratio in the range of about0.5-1.5, and a plurality of blade rows of such more easily folded bladesmay combine to provide a similar hydraulic efficiency as a longerserpentine blade. Further, in blood pumping applications, the use of along serpentine blade may lead to separated flows, leading tothrombosis, which can be avoided using multiple blade rows.

Hence, impellers according to some embodiments of the present inventionmay have multiple separate sets of blades, rather than a long,continuous helical blade. A continuous long helical blade is difficultto fold up against the hub, and by splitting a long blade into two orthree shorter sections, the blade can be more easily folded into acylindrical volume or space and subsequently deployed when desired.

An impeller according to the present invention may include at least twoblades arranged about the circumference of the hub in a first blade row.The at least two blades may be positioned approximately 360/N° apartfrom one another about the circumference of the hub, where N representsthe total number of blades in the first blade row. The impelleralternatively may include a plurality of blades arranged in at least twoblade rows, with each blade row including at least two blades. The atleast two blades in the first row of blades may be positioned 360/N₁°apart from one another about the circumference of the hub and the atleast two blades in the second row of blades may be positioned 360/N₂°apart from one another about the circumference of the hub, where N₁represents the total number of blades in the first row and N₂ representsthe total number of blades in the second row. N₁ and N₂ may be the sameor different. The first and second rows of blades may becircumferentially offset relative to one another by 360/2N₁°.

Preferably, the number of blade rows is two or three. The blade rows maybe interleaved (overlapping along the axial direction), which canincrease performance but may increase the diameter of the impeller inthe stored configuration. If blades are interleaved, they will tend tofold on each other in the stored configuration, increasing the storeddiameter of the impeller.

To minimize the stored diameter of the impeller, for example for bloodpumping applications, the blade rows are preferably spaced apart alongthe hub, proximate to each other but not interleaved. For example, thespacing between blade rows may be less than the axial extent of eachblade row along the hub axis. A larger blade row spacing allows shearwakes to decay between blade rows, but lengthens the impeller, making itmore difficult to move the impeller along a curved path, such as alongblood vessels to a desired location within a patient.

Blade rows may also be clocked relative to each other, the clockingbeing an angular displacement between corresponding blades of each bladerow, and in particular, an angular displacement between the trailingedge of a blade in one row and the leading edge of the correspondingblade in the next row. For example, an impeller may have at least afirst blade row and a second blade row, the first and second blade rowsincluding a similar number of blades. The blades of the first blade rowmay be angularly offset (clocked) relative to the corresponding bladesof the second blade row. In blood pumping applications, the angularoffset can be adjusted to reduce hemolysis. Blades may be clocked sothat the leading edge of a following blade does not reside in the wakefrom a leading blade, and the clocking may be clockwise orcounterclockwise to achieve this. Blade rows may be clocked relative toeach other to avoid tandem blade effects, where the following bladeresides wholly in the boundary layer or wake of the leading blade, so asto reduce shear stresses.

Other Blade Parameters

The amount of lean or tilt of the blades may be adjusted according tothe blade stiffness. The blade lean may be a forward lean (toward thepressure face) of between about 30° and about 60°. A forward lean bladetends to deform so as to increase the angle of incidence of the fluid atthe tip, and thus increase the load on the blades. Conventionalpropellers use a backward lean blade which tends to unload the blade tipunder structural deflection. Hence, the use of a forward lean isunusual. Forward lean of flexible blades may be used to minimize the gapbetween a blade tip and the inside surface of a conduit in which theimpeller operates. A backward lean may make it more difficult to controlthe size of the gap.

The twist pitch angles of the blades also may be variable, for examplefor impeller operation in a conduit. The blade deviation angle may be inthe range about 15 to 30 degrees to assist impeller operation at lowReynolds number operation (for example, less than 50,000 for the bladetip chord) within a conduit, hence reducing hemolysis in blood pumpapplications. The pitch at the tip, measured relative to acircumferential direction, can be appreciably less than the pitch at theroot, to match slower fluid flow within a boundary layer near the insidesurface of the conduit. Hence, the blade of an improved impeller foroperation within a laminar flow profile within a cannula has a bladetwist, the pitch being approximately matched to the flow profile, theblade tip having a smaller pitch angle than the blade root. The blademay have a slightly humped appearance in a region of relatively rapidchange in blade pitch.

For external flow applications (not in a conduit), the twist in theblade pitch may be in the opposite direction as any boundary layer willtend to be closer to the hub.

The root of a serpentine blade exhibits a geometric characteristic knownas camber, the curvature of the airfoil section if laid out on a flatsurface. By dividing a single long blade into two or more sections, thecamber of the resulting partial sections can be limited, for example tovalues of less than 10%, for example 5%. In the latter case, thedeviation of any section from a straight line will be on the order offive percent of the section chord length. For example, a serpentineblade having a camber of 15% may be divided into three sections, each ofwhich will be substantially linear and more easily folded. The resultwould be an impeller having three blades, arranged in three blade rows,with performance similar to that of the serpentine blade. The wrap angle(from the leading edge to the trailing edge) can be limited to a maximumof about 30 degrees. Hence, blade rows may each contain substantiallytwo-dimensional blades (compared with a single pumping efficiency), theblades serpentine blade of similar more readily folding against the hubthan would a serpentine blade.

The modulus of the blades may be lower for deforming the blades to thestored configuration of the impeller, which may correspond, for example,to strains of 100% 200%. In representative examples, the modulus for thelarger blade deformations in the stored configuration may be about tentimes less than the modulus for operational stresses. For example, theimpeller may include blades formed from a material having a flexuralmodulus of about 10,000 psi for operational stresses and about 1,000 psifor storage deformations.

FIG. 2A shows an impeller 200 in a deployed configuration, the impellerincluding a hub 210 and a plurality of blades 212. Impeller 200 has aradius R 1 in the deployed configuration, as measured from the centrallongitudinal axis of hub 210 to the outermost blade tip. The deployeddiameter is twice the deployed radius, and is the diameter of a circledescribed by the blade tip as impeller 200 rotates around thelongitudinal axis of hub 210. Also shown is a wall of a conduit 250through which fluid flows relative to impeller 200. Impeller 200 may beused as an axial pump, to pump fluid through conduit 250. Alternatively,impeller 200 may be used as a motive force provider for a vehicle. Forexample, the impeller may power a boat, such as a jet-boat, or otherwater craft. In such example, conduit 250 could be a tube immersed inthe water surrounding the vehicle, or there may be no conduit at all.

FIG. 2B shows impeller 200 in a stored configuration, with blades 212folded or otherwise deformed towards hub 210 and held in this storedconfiguration by a storage housing 260. The radius R₂ of storage housing260, which in this case defines the stored diameter of impeller 200, isappreciably less than the radius R₁ of the deployed impeller shown inFIG. 2A. A deformed impeller blade 212 may contact the inside surface ofstorage housing 260 at one or more locations.

In embodiments of the present invention, the flexible blades 212 ofimpeller 200 can be folded or otherwise radially compressed such thatthe maximum diameter of the impeller in the stored configuration isapproximately half, or less than half, the diameter of the impeller inthe deployed configuration. Referring to FIGS. 2A and 2B, thiscorresponds to R2≈≤(R1/2). A ratio of R2≈≤(R1/2) is useful for bloodpump applications, allowing a blood pump to deploy to a diameter ofbetween about 6 millimeters and about 7 millimeters within a human body,while being non-surgically inserted with a diameter of between about 3millimeters and about 4 millimeters. Other diameter ratios are usefulfor other applications.

Impeller Deployment within a Conduit

FIG. 3 is a schematic view illustrating the deployment of impeller 200.Impeller 200 has hub 210 and blades 212, and is retained in the storedconfiguration by storage housing 260. Storage housing 260 may be a tubein which impeller 200 is stored prior to deployment. A drive shaft 230is used to rotate impeller 200. The figure also shows an optional guidewire 280 within rotating drive shaft 230 which can be used to positionimpeller 200 at a desired location. The rotation of drive shaft 230 mayalso assist in deploying impeller 200, for example through twisting theimpeller out of storage housing 260 if the inner surface of the storagehousing has a threaded texture.

On the left of the figure, conduit 250 is shown into which impeller 200is deployed for operation in its larger deployed configuration. Conduit250 may represent any structure through which a fluid may flow relativeto impeller 200, such as a tube, catheter, cannula, or body vessel suchas a blood vessel.

Impeller 200 contained within its storage housing 260 may be deployedwithin a conduit, such as a utility pipe (water, gas, sewage, and thelike), body vessel (such as a blood vessel), portion of a thrust unitfor a vehicle, or other structure through which a fluid may flow. Theimpeller can be conveyed to a desired location within the conduit in astored configuration, and then deployed to the deployed configuration.Impeller 200 can be deployed by urging the impeller axially out ofstorage housing 260, for example using drive shaft 230 attached to theimpeller. The impeller then unfolds into the deployed configurationusing the stored potential energy of blades 212 in the storedconfiguration.

The stored configuration facilitates conveyance of impeller 200 to thedesired location, enabling it to be passed through openings which aresmaller than the diameter of the impeller in the deployed configuration.To remove impeller 200 from the conduit after use, the impeller may beradially compressed back into the stored configuration, for example byurging the impeller back into storage housing 260, such as by re-foldingflexible blades 212 against hub 210. The stored impeller may then beremoved from the use location through an access hole having a dimensionless than the diameter of the impeller in the deployed configuration.Hence, impeller 200 can be inserted in the stored configuration througha relatively small entrance hole into a conduit 250 of larger diameter.

Although storage housing 260 is described above as a tube from whichimpeller 200 may be deployed by axial movement out of the storagehousing, that need not be the case. Rather, storage housing 260 mayitself be expandable or have an expandable portion, as described below.Expansion of storage housing 260 would allow impeller 200 to deploy,such that the impeller would not need to be pushed axially out of thestorage housing to achieve the deployed configuration. Thus, withreference to FIG. 3, conduit 250 may represent storage housing 260 in anexpanded condition.

Winglets

Impellers according to the present invention may include at least oneblade having a winglet. In some embodiments, all blades within a bladerow may include such a winglet; other blades in the impeller mayor maynot include a winglet. A winglet may improve hydrodynamic performance ofthe impeller in the operating state, and may also reduce shear stressesthat exist within the fluid being pumped. As a result, when the fluidbeing pumped includes biological structures such as cells, thedegradation of such structures by the pumping action may be reduced.

An impeller blade typically has a pair of opposed faces: a pressure faceinducing relative motion of the fluid through pressure as the bladerotates through the fluid; and a suction face inducing fluid motion bysuction. Typically, the pressure and suction faces are not planar, butrather are curved in the same general direction to define an airfoilshape. The blade also has a leading edge cutting through the fluid asthe blade rotates, a trailing edge, and an outer edge (which may also bereferred to as a blade tip or distal end of the blade). A winglet mayextend in the direction of motion of the impeller (from the pressureface of the blade), in the direction opposite the direction of motion(from the suction face of the blade), or in both directions.

FIGS. 4A and 4B show perspective and cross-sectional views,respectively, of a blade 212 of impeller 200 having a winglet 222 at itsdistal end. FIG. 4A shows the cross-section of the blade 212 where itjoins winglet 222 as a dashed line, the winglet significantly enlargingthe cross-section of the blade at its distal end. FIG. 4B shows blade212 and winglet 222 in cross-section, in which the winglet and bladeform an approximate T-shape. As shown in FIG. 4B, the suction side ofthe blade is on the right, and the pressure side is on the left. Ifblade 212 has a thickness between the pressure face and the suction faceat the distal end of the blade, winglet 222 may have a width betweenabout 1 and 3 times the distal thickness of the blade, measured in adirection parallel to the blade rotation direction. If blade 212 has achord length, winglet 222 may have a length approximately equal to thechord length.

Winglets 222 are preferably aerodynamically smooth shapes having leadingedges where flows impact the edges of the winglets, and trailing edgeswhere flow is discharged from the winglet surfaces. Winglets 222preferably have smooth aerodynamic cross-sections, generally in thedirection of the mean flow, which is parallel to the flow directionalong the blade tip surfaces. FIG. 5D shows possible leading edgegeometries, including a radius edge 240, a sharp edge 242, and chiseledges 244 and 246.

Where impeller 200 rotates within a conduit 250 for fluid flow, thedistal end of blades 212 in the deployed configuration, either with orwithout a winglet 222, may be located proximate to the interior surfaceof the conduit, so as to define a tip gap 255 between the blade distalend and the inner surface of the conduit. The tip gap 255 may be about10 to SO percent of the maximum thickness of the distal end of theblade. In such circumstances, appropriately shaping the tips of theblades, such as by providing the tips with winglets 222, can improve thequality of the flow field and reduce shear stresses. As shown in FIG.4B, the winglet 222 may be proximate to the inner surface of conduit250, a configuration which may be used as a hydraulic bearing for animpeller 200.

For blood pump applications, simulations have shown that most hemolysisoccurs at the blade tip, and that winglets lowered the hemolysis.Alternatively, the tip shape may be rounded to reduce hemolysis. Arounded blade tip reduces flow separation and turbulence at the tipcompared with a squared-off tip, and winglets further reduce flowseparation and hence turbulence in the wake of the blade tip as it movesrelative to the fluid. By using a winglet 222, the gap 255 between thetip of blade 212 and the inner surface of conduit 250 may be increasedwhile retaining the performance of an impeller having a smaller tip gapbut no winglets. This effect is analogous to retained lift near the endof winglet-equipped airplane wings. Fluid head losses are minimized fora tip gap in the range of about 0.10-0.15 times the maximum thickness ofthe distal end of the blade for blades without winglets, this rangebeing expected to reduce hemolysis by reducing shear stresses in gapflows. Winglet-equipped blades show minimum fluid head losses at a tipgap of about 0.25-0.30 times the maximum thickness of the distal end ofthe blade. An increased tip gap reduces shear stresses for impelleroperation in a conduit, and for blood pumping applications hemolysis isreduced compared with the use of a smaller tip gap.

FIG. 5A shows a suction side winglet 262 extending from the outer edgeof the suction face of blade 212. This is a view from the leading edge,in cross-section, so that the blade rotates towards the direction ofviewing. FIG. 5B shows a pressure side winglet 264 extending from thepressure face of blade 212. The parameters may be similar to the suctionside winglet. The function of the pressure side winglet is to reduceflow through the gap 255. There is less effect of creating ahydrodynamic bearing, but the pressure side winglet “scrapes” lowmomentum fluid off the inner surface of the conduit 250 and preventsthis fluid from entering gap 255 and subsequently being used in the coreof a tip vortex. This can reduce shearing stresses in the bulk of thefluid flow.

FIG. 5C illustrates a combined winglet 266 extending from the outer edgeof both the pressure and suction faces of blade 212. Embodiments of thepresent invention include the configurations shown in FIGS. 5A-5C.Numerical methods can be used to design the winglet configurations.Where the blade chord lengths are long and the blade has a significanthelical extent, the geometry and shape of the blade tip and the wingletcan become complex.

FIGS. 6B-6D further illustrate winglet configurations, the bladesupporting the winglet retaining the same shape in these examples. FIG.6A illustrates the outer edge shape 270 of a blade 212 not having awinglet.

FIG. 6B shows a pressure side winglet extending from the outer edge ofthe pressure face of blade 212, extending over portion 272. The portion270 of the winglet corresponds to the original outer edge shape of theblade shown in FIG. 6A.

FIG. 6C shows a suction side winglet, the portion 274 extending from theouter edge of the suction face of the blade, and the portion 270corresponding to the original outer edge shape of the blade. In someembodiments of the present invention, the pressure side of the bladewill have a radius of approximately ⅓ to ½ the blade thickness or width.The extent of the winglet may be from ½ to 3 times the blade thickness.A thickness approximately equal to the blade thickness is shown. Thewinglet is mostly positioned to the downstream half of the blade asshown. The purpose of this is to create a hydrodynamic bearing in whichthe outer face of the winglet is in close proximity to the inner surfaceof the conduit in which the blade is operating. With such configuration,the flow in the gap 255 between the winglet and the inner surface ofconduit 250 is reduced in strength, and a tip vortex is less likely toform. This reduces shearing stresses in the fluid. Gap 255 can bebetween about 10 percent and about 25 percent of the base blade maximumthickness, and is an area that is mostly parallel to the fluid conduit250. It can be a cylindrical, conical or curved side cylinder where theradius is a function of the axial position of the blade element.Parameters for pressure side winglets and combined winglets (describedbelow) may be similar.

FIG. 6D shows a combined pressure side and suction side wingletextending from both the pressure face and the suction face of the blade,the portion 276 extending from the suction face, the portion 278extending from the pressure face, and the portion 270 corresponding tothe original outer edge shape of the blade.

Features to Aid Stored Configuration

Referring to FIGS. 7A and 7B, impeller 200 may have one or morestructural features which aid achievement of the stored configuration.Such structural features may include one or more indentations proximateto the blade root to facilitate deformation of the blade into the storedconfiguration. An elongated indentation, such as a trench 282, may beformed in the hub 210 of impeller 200 proximate at least part of theproximal end of the blade (the blade root, where the blade joins thehub). Trench 282 may be formed adjacent to one or both of the suctionface and the pressure face of blade 212. Preferably, trench 282 isformed in hub 210 parallel with and adjacent to the proximal end of theblade.

The structural features may facilitate movement of the distal end ofblade 212 towards hub 210. For example, a trench 282 around some or allof the blade root can help reduce internal mechanical stresses in blade212 when the blade is in the stored configuration, for example foldedagainst hub 210.

In some embodiments, blade 212 may have a cross-section in the shape ofan airfoil and the indentation may be a curved trench formed in theimpeller hub 210 parallel to the proximal end of the blade. Otherstructural features which may aid achievement of the storedconfiguration include hinges (such as living hinges) with one or moreindentations or cuts, not shown but known in the art, in the blade 212and/or hub 210; forming a portion of impeller 200 proximate the bladeroot from a more easily deformable material; and the like.

The indentation may also be referred to as a “dillet,” and may includeany undermining of the blade root. A dillet may be a trench proximatethe blade root, for example having a depth between about 0.5 and about1.5 times the blade width, and/or a width of a similar size range. Adillet can facilitate folding of the impeller blade towards the hub toachieve the stored configuration.

The dillet may also reduce fluid shear stress and flow vortices in afluid moving relative to impeller 200 as the impeller operates. In bloodpumping applications, lower shear stresses lead to reduced hemolysis ofthe blood.

Hub 210 may have dillets proximate both faces of blade 212, one dilletfacilitating folding of the blade (depending on the direction the bladeis folded towards the hub), both dillets reducing the formation of aroot junction vortex and hence reducing hemolysis in blood pumpingapplications. The dillet may be a horseshoe dillet, for exampleapproximating the shape of a horseshoe vortex that would otherwise format the blade root. Hence, dillets may be provided to reduce shearstresses, even for impeller blades that are not folded.

Blade Materials and, Modulus

Impeller 200 may be in the form of impeller body, including hub 210 andone or more a unitary blades 212 formed from a single material.Alternatively, blades 212 and hub 210 may be formed from differentmaterials. Preferably, blades 212 are flexible so that they can bedeformed towards hub 210 in the stored configuration. Blades 212 may beformed in any way that allows expansion from a stored configuration to adeployed configuration, the deployed diameter of impeller 200 beinglarger than its stored diameter.

Blades 212 may be formed from any material that permits the achievementof a stored configuration in which the blades are folded toward hub 210.In that regard, the blades may be formed from a rubbery, elastic orother material having sufficient resilience to expand when the bladesare no longer held in the stored configuration, such as when impeller200 is deployed from a storage housing. The blades may be formed frompolymer materials, such as polyurethane or other polymers havingsuitable elasticity properties. For medical devices such as blood pumps,biocompatible polymers are preferred. The average molecular weight maybe chosen within a given range to obtain desired properties.Alternatively, the blades may be formed from other flexible polymers,from expandable foam optionally with a skin, or from other compressibleor deformable materials including shape-change or shape-memorymaterials, and metals. Blades 212 may be formed with both asubstantially rigid portion and a flexible portion, the blades beingdeformed towards hub 210 by deformation of the flexible portion. Theflexible portion may include a hinge, such as a living hinge, a narrowedregion, a material which is different from the material of the rigidportion, or other configuration.

Blades 212 and (optionally) hub 210 may be constructed of a low moduluspolymer, for example a low flexural modulus polyurethane (this termincludes polyurethane ureas, which were used to form impellers accordingto the present invention). Impeller 200 may be a unitary structure, withthe blades and hub formed as one from the same material, for example bymolding a polymer.

In some examples, blades 212 may have a stiffness approximating that ofa thick rubber band. In such embodiments, the blades will have somestiffness, but will deform under operating loads. For example, thematerial forming impeller 200 may be chosen so as to have a linearmodulus at operational stresses, allowing predictable deformation of theblades under load, and a non-linear modulus at the higher stresses usedto deform the blades into the stored configuration.

Impeller 200 may have blades 212 formed from a polymer, such as apolyurethane, having a flexural modulus (for operational stresses)between about 3,000 psi and about 30,000 psi, more preferably betweenabout 5,000 psi and about 20,000 psi, and still more preferably betweenabout 7,000 psi and about 10,000 psi. The modulus for operationalstresses corresponds to deformations of the impeller during operation,which in some examples may correspond to strains of approximately 5%.The blade thickness may be reduced when using higher modulus materialsto achieve the desired flexibility.

Impeller blades 212 may, for example, occupy as much as 95% of thecompressed volume of impeller 200 when the impeller is in the storedconfiguration.

FIG. 8 is a stress-strain curve for a non-linear material that can beused to form impeller 200 according to the present invention. The left(low stress) filled circle corresponds to an impeller operating point(stress and strain under operating conditions) and the right (highstress) filled circle corresponds to the impeller stored configuration.The stress/strain relationship is approximately linear at the impelleroperating point, so that deformations due to operational stresses can beaccurately predicted by numerical modeling. The stored configuration, inwhich blades 212 are folded against hub 210, is within a high strainnon-linear portion of the curve. This allows the stored configuration tobe achieved without passing the material tensile failure point, and alsoreduces the stresses necessary to achieve the stored configuration. Inexample impellers 200, the maximum material elongation in the storedconfiguration is about 75 percent.

Preferably, a non-linear material, such as one having thecharacteristics of FIG. 8, is used for blades 212. This allows the bladematerial to be relatively stiff at operating loads, and relativelyflexible at higher strains, such as when blades 212 are folded in thestored configuration. For example, the strain might be 1-10 percent atoperating loads and 75 percent while folded, and the stress/strain curvemay correspond to a higher modulus (e.g., 10,000 psi) at operatingloads, and to a lower modulus (e.g., 1000 psi) at the higher loadsassociated with folding. The stress-strain curve may have twoapproximately linear regions with a sharp change in slope between theoperating point strain and the folded strain.

Impellers 200 may be fabricated from commercially available polyurethanepolymers, for example having a modulus between about 5,000 psi and about10,000 psi. Example impellers according to the present invention werefabricated as unitary bodies (including hub and blades) from elastomericpolymers. Example materials used include Conathane™ TU-901 (CytecIndustries, Inc., West Paterson, N.J.), which had a modulus of about10,000 psi for operational deformations; Conathane™ TU-701 (modulus ofabout 7,000 psi), and Hapflex™ 560 (Hapco Inc, Hanover, Mass.), whichhad a modulus of about 5,000 psi. However, other polyurethanes, otherpolymers or other materials may be used.

A polymer impeller 200 retained in the stored configuration forexcessive time periods may not properly deploy, for example due to creepor electrostatic welding between adjacent polymer surfaces. Preferably,impeller 200 is retained in the stored configuration only as long asnecessary to insert the impeller to a desired location. Hydrodynamicstress and forward lean may be helpful both to deployment and overcomingany hysteresis effect.

Impeller 200 may deploy from the stored configuration due to storedpotential energy associated with blade deformation towards hub 210 inthe stored state. However, other stored potential energy may be used(for example, using shape memory materials). Depending on theapplication, external energy may be conveyed to impeller 200, such asheat (for example, electrical heating of a wire or other structure),centrifugal forces, electromagnetic forces, gas jets, and the like toassist the deployment of the impeller.

Impeller Fabrication

Impeller 200 may be fabricated using molding, investment casting (forexample, using a hard wax master), stereolithography, milling, or othertechniques. Impellers 200 of the present invention have been fabricatedusing a flexible mold to avoid the presence of significant mold partlines.

Very small impellers, approximately 6-7 mm in diameter in the deployedconfiguration, may be fabricated from a polymer (such as a polyurethane)and extracted from a precision mold. This allows production of impellersat very low cost. The flexible blades 212 allow the impeller to beextracted from a mold without becoming mold-locked, and allow the use ofone-piece molds, instead of multi-part or split molds. This can beadvantageous for producing impellers designed for pumping bio-fluids.

Impeller Optimization

Blade shapes can be optimized using standard computational fluiddynamics analysis (CFD). If the impeller material is not flexible, thereis no deformation of the impeller when rotating. An improved method ofoptimizing an impeller 200 formed of a flexible material is to optimizethe deployed configuration under operational stress (which may be termedthe operational configuration). The impeller can be designed so that theoperational configuration is optimized, which is not necessarily thesame as the deployed configuration under no loading. A structuralcomputation allows the determination of deformation under the load ofoperational stresses. Hence, impeller 200 may have flexible blades 212that deform into an optimized hydrodynamic shape when rotating andoperating under design load conditions.

The impeller blade 212 can be designed so as to minimize destruction ofdelicate particles (such as emulsion droplets, suspensions, and thelike) within a biological structures fluid. A CFD model such as cells,may be used to simulate the through an intermediate be used todestruction shear stresses experienced by particles passing simulatedimpeller. Time integrations of shear stresses experienced by theparticles may provide an estimated probability of cell in a biomedicalapplication. A split blade design, in which there are a plurality ofblade rows such as discussed above, reduces the residence time in whichcells remain in intermediate shear stress regions, allowing anadvantageous reduction in cell or other particle destruction comparedwith a single long helical blade.

The impeller blade(s) 212 may deform during operation, and the optimumconfiguration of a blade may be achieved only upon deployment androtation. For example, the optimal, design configuration of blade 212may be achieved only with operational stresses. Hence, blade deformationin operation, due to flexibility of the blade, need not lead to reducedperformance. Successful operation can occur even when impeller 200exhibits significant deflections from a manufactured shape. The impellercan be manufactured with allowance for the deflection included in thedesign. The configuration of an impeller operating at a predeterminedrotation rate, or within a predetermined operating range, can beoptimized. Hence, in further embodiments of the present invention, theoperational configuration of the impeller, including deformation due tooperational stresses, is optimized.

For blood pump applications, CFD optimization may be used to minimizeflow velocity over blade surfaces (Reynolds number), vortex formation,flow jets, root junction flows, and to avoid formation of separatedflows that may lead to thrombosis.

Reynolds Number

An impeller 200 according to the present invention can operate in a lowReynolds number conduit flow, where the conduit boundary layer comprisesa majority of the flow in the conduit. The Reynolds number is theproduct of blade velocity and chord length, divided by the fluidviscosity. The Reynolds number varies with radius, and generally refersto the tip (distal end) of the blade unless otherwise stated. Forexample, the Reynolds number for operation of a conventional propellermay be on the order of millions, so that there is a turbulent flowtransition as the fluid passes over the blade.

Impellers 200 can be used with flows of small Reynolds number, less than30,000 for the blade tip, for example the pumping of relatively viscousfluids at low velocity or flow rate. Impellers according to the presentinvention may operate with blade chord Reynolds numbers of between about1,000 and about 30,000, preferably between about 2,000 and about 20,000,and more preferably between about 5,000 and about 20,000. The operationat such low Reynolds numbers corresponds to substantially laminar flowof the fluid over the blades. The reduced turbulence leads to reducedshear stress, and reduces hemolysis in blood pumping applications.

Impellers 200 may also be used for flows of larger Reynolds numbers,such as from 100,000 to several million. Impeller diameters can be inthe range of several millimeters (or less) to several meters, dependingon the application.

For operation in a conduit 250, impeller 200 may be located within afully developed laminar flow profile, for example at a distance of about10-15 times the conduit diameter from the conduit inlet.

A plurality of blade rows may be operated at a lower Reynolds numberthan a single longer serpentine blade having similar hydraulicefficiency. Each blade row may be separately optimized, for example toobtain substantially laminar flow. The blade rows may be clockedrelative to one another to reduce hemolysis in blood pumpingapplications. Flow separations leading to thromboses may be avoided.Further, each blade row-may include a different number of blades, forexample 1, 2 or 3 blades. Hence, a plurality of blade rows may be usedto reduce hemolysis in blood pumping applications compared to a singleserpentine blade, while retaining similar or improved efficiency. Forexample, axial heads may be obtained similar to mixed flow pump heads.

For example, in an impeller 200 operated within a cannula as a bloodpump, such as blood pump 600 described below, the blade chord Reynoldsnumber of the first blade row was about 12,600, and was about 15,800 forthe second blade row. This suggests that the flow was substantiallylaminar. In this case, the blades 212 may not exhibit a transition toturbulent flow over the blade surface (where the shear stress suddenlyjumps to a higher value), which for blood pump applications leads tolower hemolysis.

Hemolysis

Hemolysis refers to the breakdown or destruction of red blood cells,releasing the hemoglobin contained therein. For blood pumpingapplications, hemolysis for a given impeller 200 can be estimated usingequations known in the literature, and parameters discussed hereinadjusted to reduce hemolysis.

Both the hemolysis and platelet activation analyses can be conducted bycombining the model of Garon and Farinas, “Fast Three-DimensionalNumerical Hemolysis Approximation,” Artificial Organs, 28(11):1016-1025(2004), with empirical correlations of Giersiepen et al., “Estimation ofShear Stress-related Blood Damage in Heart Valve Prostheses—In vitroComparison of 25 Aortic Valves,” International Journal of ArtificialOrgans, 13(5), 300-306 (1990). Giersiepen proposed empiricalcorrelations for LDH-release by platelets:

${{\frac{\Delta \; {LDH}}{LDH}(\%)} = {3.31 \times 10^{- 6}\mspace{11mu} t_{\exp}^{0.77}\mspace{14mu} \tau^{3.075}}},$

and for Rb-release by red blood cells:

${{\frac{\Delta \; {Hb}}{Hb}(\%)} = {3.62 \times 10^{- 5}\mspace{14mu} t_{\exp}^{0.785}\mspace{14mu} \tau^{2.416}}},$

there t is in seconds and τ is in Pascals. The Garon and Farinas model,in short, provides a framework for any damage model of the form

D=γt _(exp) ^(β)τ^(α)

by calculating the net flux of the parameter through a volume. So

D = (D_(I))^(β)$D_{I} = {\frac{I}{Q}{\int\limits_{v}{\sigma \; {dV}}}}$ andσ = (γ)^((1/β))  τ^((α/β)).

The variable 1 is the scalar form of the stress tensor referred to asthe Von Mises Criterion and is specified as:

τ=[1/2[σ₁−σ₂)²+(σ₂−σ₃)²+(σ₃−σ₁)²]]

where cr1, CT2 and CT3 are the principal stresses.

g/IOOL) is then: According to Garon and Farinas, the normalized index ofhemolysis (in NIH=100 Hb D, where Hb is the hemoglobin concentration ingrams per liter, and the modified index of hemolysis (in parts permillion) is:

MIH=10MIH=10⁶ D.

By analogy, the platelet activation rate would be given by PIA=PTD,where both PI and PIA are platelet concentrations, for example inthousands per microliter.

Garon and Farinas only considered the hemolysis rate and only in laminarflow, and so the principal stresses are in reference to the laminarviscous stress tensor only. Garon and Farinas made no reference to howturbulent flow would be accommodated, but that discussion does takeplace in other research. Arvand et al. [“A Validated Computational FluidDynamics Model to Estimate Hemolysis in a Rotary Blood Pump,” ArtificialOrgans, 29(7):531-540 (2005)] actually advocated neglecting the Reynoldsstress term in turbulent flow simulations in order to avoid “numericallycaused variety” of the scalar form of the shear stress used in thehemolysis regression model, but the more conventional approach has beento use the effective shear stress (laminar plus Reynolds stress). See,for example, Gu et al., “Evaluation of Computational Models forHemolysis Estimation,” ASAIO Journal, p. 202-207 (2005).

Both approaches may be taken and compared for both and plateletactivation analyses. The flow is simulated using computational fluiddynamics, the shear stress is determined from the determinedthree-dimensional flow data, and the hemolysis may then be determinedfrom the shear stress distribution.

Stored Configuration and Deployment

Impeller 200 may be stored in a storage housing, such as storage housing260, transported to a desired location in the stored configuration, and,once at the desired location, deployed into a deployed configuration.Rotation of the impeller then induces fluid flow at the location. Forexample, the impeller in the stored configuration may have a diameterapproximately equal to or less than half the diameter of the impeller inthe deployed configuration, the diameter of the stored configurationbeing generally defined by the inner diameter of the storage housing.The storage housing may be any assembly which acts to hold the impellerin the stored configuration, and may comprise a tube, sleeve, or similarstructure inside which the impeller is stored prior to deployment.

In the stored configuration of impeller 200, blades 212 may be folded intowards hub 210, or otherwise deformed or reconfigured so as to presenta reduced diameter compared with the deployed configuration. Impeller200 may be held in the stored configuration by storage housing 260. Inthe stored configuration, the distal ends of blade 212 are closer to hub210 than in the deployed configuration, and the stored diameter of theimpeller can be significantly less than its deployed diameter. Storeddiameters of about one-half of the deployed diameter or less areachievable.

The storage housing need not have a fixed diameter, as does storagehousing 260, but may include a non-expandable portion, in which impeller200 is stored, and an expandable portion, into which the impeller can bemoved for deployment. Impeller 200 may then deploy within the expandedportion of the storage housing. The expandable portion of the storagehousing may also have a stored configuration. For example, the diameterof the expandable portion in the stored configuration may beapproximately half or less of its diameter in the expanded state.Alternatively, the entirety of the storage housing may be expandablesuch that impeller 200 does not have to be moved axially for deployment.

Storage housings that are expandable or that include expandable portionsmay be held in a compressed state by a retainer sleeve, described below.Impeller 200 may be stored in a compressed configuration within thestorage housing when the storage housing is retained in the compressedstate. However, once the retainer sleeve is removed from that portion ofthe storage housing in which impeller 200 is located, the storagehousing and impeller can expand to their expanded or deployedconfigurations. Impeller 200 may be deployed by urging the impellerblades 212 out of the confines of the storage housing, for example bypulling the retainer sleeve away from that portion of the storagehousing overlying the impeller. In some embodiments, the retainer sleevemay be expanded in situ so as to allow impeller 200 to achieve thedeployed configuration. Other methods of deploying impeller 200 will beclear to those skilled in the art.

Impeller 200 may be deployed by various methods. For example, thestorage housing may be expandable so as to have an expandedconfiguration when impeller 200 is in the deployed configuration, and acompressed configuration when the impeller is in the storedconfiguration. In such embodiments, the storage housing acts to radiallycompress impeller 200 in the stored configuration and allows theimpeller to deploy when the storage housing expands. Alternatively, forstorage housings that do not expand, impeller 200 may move axially outof the storage housing.

FIG. 9A illustrates an impeller 300 in a stored configuration, showingblades 332 and 334 and hub 310. Blades 332 and 334 are kept foldedagainst hub 310 by the housing 360. FIG. 9B shows impeller 300 pushedstorage housing 360 and in the deployed configuration. Blades storageout of In the embodiment shown, impeller 300 has two rows of blades, asis seen more clearly in the deployed configuration, the first rowincluding blades 332 and the second row including blades 334.

FIG. 10 shows an impeller 400 including a hub 410 and a plurality ofblades shown in both the deployed and operating configurations. Thefigure allows comparison of the deployed configuration under no loadwith the deployed configuration under operational stresses, when theimpeller rotates at the operational rotation speed. In the deployedconfiguration under no load, the blades assume a first shape indicatedby reference numbers 462A, 464A and 466A. When rotating in a fluid, theblades deform to an operational configuration indicated by referencenumbers 462B, 464B and 466B. Impeller 400 may be designed so that theflexible blades deform into an optimized hydrodynamic shape whenrotating and operating under design load conditions.

In general, the blades deflect forward as the lift on the blades is suchthat they create thrust, a force directed towards the left side of thefigure, moving the blades toward the left side of the picture. Theleading edge of the second blade row is obscured. In this example, thereare two blade rows, each with two identical blades. For example, thefirst blade row includes blade 462, shown in an operating configurationat 462B and under a no load condition at 462A. The leading edge of eachblade transitions smoothly into the trailing edge at the maximum bladeradius. For a hub and blades formed from the same polymer, simulationsshowed that the hub also deflects slightly in a rotational manner, withthe second blade row rotated at the root compared to the first bladerow.

FIGS. 11 and 12 illustrate optimization for fluid shear stress for anexample impeller having a design similar to impeller 400 shown in FIG.10. The distal ends of the impeller blades move proximate to theinterior surface of a cylindrical conduit such that the tip gap betweenthe blade distal end and the inner surface of the conduit is about 10 to50 percent of the maximum thickness of the distal end of the blade.

The curves are double normalized, the design point values both being1.0, the scales being read as percent of design flow and a factor timesthe value of stress at the design point. For example, FIG. 11illustrates that at 70 percent of the design flow, the shear stress is1.3 times the value at the design condition. FIG. 12 shows that makingthe tip gap smaller than the design value makes the shear stress higher,whereas making the gap bigger than the design value reduces the shearstress by a smaller factor. Therefore, the fluid shear stress can bereduced to lower hemolysis in blood pumping applications, withoutsignificantly compromising pumping efficiency.

FIG. 13 illustrates an impeller 500 including a hub 510, and two rows ofblades having two blades each. The first row includes blades 582 and584, and the second row includes blades 586 and 588. Impeller 500 hashighly curved leading and trailing edge lines where the blade pitchangles are adjusted for local values of relative flow angle. Impeller500 was designed for operation inside a conduit, such as a cannulahaving a laminar flow profile, the flow rates being lower near theinside surface of the conduit. This illustration shows the designelements of a low Reynolds number impeller for use in a left ventricularassist device (LVAD) in which the thickness of the boundary layer on thefluid conduit walls approaches the diameter of the conduit. The Reynoldsnumber for a blood pump application was determined to be in the range of10,000-20,000.

Expandable Impeller and Expandable Cannula

In some embodiments, an expandable impeller is used together with acannula which may or may not have an expandable portion. If the impelleris not stored in an expandable portion, the impeller must be movedaxially for expansion to its deployed configuration. If the impeller isstored in an expandable cannula or in an expandable portion of acannula, the impeller expands into its deployed configuration with theexpansion of the cannula. This combination may be used in improved bloodpumps, such as an improved left ventricular assist device (LVAD).

For example, a cannula may be provided that has expandable andnon-expandable portions, and the impeller may be stored within, orproximate to, the non-expandable portion. The impeller can be urged outof the non-expandable portion of the cannula into an expanded portion ofthe cannula. The stored potential energy within the flexible blades ofthe impeller would then induce self-deployment of the impeller, and thecannula may also self-expand through stored potential energy. Theexpanded cannula then may have the role of a fluid conduit through whichfluid flows when the impeller is rotated. An example of such system isblood pump 600 described below. An expandable cannula and impeller mayboth be stored within a retainer sheath and deployed together when urgedout of the retainer sheath, as is also described below.

Applications

Impellers according to the present invention may be used for a varietyof applications, including an axial pump for a fluid (gas or liquid), amotive force for a vehicle, or other applications. Applications of theimproved impellers according to embodiments of the present inventioninclude pumps for chemical engineering, propellers for airborne ormaritime vessels, water pumps, and the like.

Impellers according to the present invention may be attached to one endof a flexible drive shaft. A torque applied to the other end of thedrive shaft is then used to rotate the impeller. The torque may beapplied by a rotating member, such as a motor.

Blood Pump

As noted above, impellers according to the present invention are wellsuited to blood pumping applications, including as a left ventricleassist device, as a right ventricle assist device, for pumping blood toother organs, and the like.

For blood pumping applications, the impeller may operate within thelaminar flow profile of a cannula flow, so that the blade pitchpreferably varies with radius to match the flow profile. An impellerwith two blade rows, such as impeller 500 illustrated in FIG. 13,feature in the second row blades that may have a groove-like takes ahelical path from the leading edge to the trailing edge. This arises dueto variations in the span wise loading, and allows an axial flow pumpusing this impeller to achieve a head rise similar to that of a mixedflow pump.

Computational fluid dynamics analysis shows that an axial bloodpump—including an expandable impeller with two blade rows was suitablefor use in a left ventricular assist device (LVAD). The impeller may becompressed and packaged into a storage housing, such as a tube, cannula,or other structure, for insertion into an object. For an object such asa living subject, the diameter of the storage housing can be about threeto four millimeters or less. Having inserted the device, the impellercan be deployed in situ into a geometry that may be about six to sevenmillimeters in diameter. The impeller then can be rotated using aflexible drive shaft coupled to a drive motor external to the subject.Such impellers may be capable of pumping 4 Lpm (liters per minute), andmore preferably 5 Lpm or greater, for example in a left ventricularassist device (LVAD).

In a representative example of such a device, the impeller may rotate atabout 30,000 RPM. The impeller may include two or more airfoil shapedblades that form an axial flow pump, and may be positioned using a guidewire. The guide wire may run within a hollow center of the flexibledrive shaft, and the hollow center may also convey saline solution orother fluid for infusion, cooling and/or lubrication purposes. The guidewire may be removed, if desired. Implantation into a living subject maybe achieved without surgical intervention through an insertion cannulahaving a diameter of about 3-4 mm. For example, a device including animpeller and a cannula may be inserted in a stored configuration throughan insertion cannula in the femoral artery, the impeller and cannulathen deploying (expanding radially) to approximately twice the storedconfiguration diameter when located at a desired location, such asproximate to the aortic valve.

For medical implantation, a drive shaft comprising a metal braid, apolymer braid or a composite material braid may be used, and the driveshaft diameter may be on the order of 1.1 to 2 millimeters, and may behollow to allow the guide wire to pass therethrough.

An impeller according to the present invention can be operated within acannula, and a flow of 5 Lpm at 100 mm Hg net pressure rise obtained(220 mm Hg across the impeller with pressure losses elsewhere). Theseparameters are well suited to blood pumping applications, for examplewithin a left ventricular assist device (LVAD).

A blood pump 600 for use in blood pumping applications, such as thosenoted above, is shown in FIG. 14. Blood pump 600 may be broken down intothree main parts as shown in FIGS. 15A, 15B and 15C. It should be noted,however, that these features may be combined to produce devicesaccording to the present invention that are intended for applicationsother than blood pumping applications.

The first part, shown in FIG. 15A, is an impeller 60S with a drive shaft630 for implementing a rotational drive to the impeller. Impeller 60Sincludes a hub 610 and a plurality of blades 612, and may have any orall of the features of the impellers described above. Hub 610 and driveshaft 630 may be hollow so as to define in blood pump 600 an internallumen 670.

The second part, shown in FIG. 15B, is a housing or cannula 625 in whichimpeller 605 resides. Cannula 625 has a storage housing 660 for impeller605 when the impeller is in a compressed state. Storage housing 660 maybe nonexpandable. Alternatively, storage housing 660 may itself beexpandable or cannula 625 may have an expandable portion for housing theimpeller when the impeller is in its operational or deployedconfiguration. Whether there is a difference in the location of impeller60S in its operating and stored configurations depends on whether theimpeller is moved axially within cannula 625 for deployment or whethercannula 625 expands in the area in which the impeller is stored.

The third part, shown in FIG. 15C, is a retainer sheath 700 which holdsat least a portion of cannula 625 in a compressed state for insertioninto a vessel of a patient. Each of these parts will be described morefully below.

The cannula 625 of blood pump 600 has a nonexpandable portion 623 at itsproximal end and an expandable portion 626 at its distal end. Theexpandable portion 626 may be flared at one or both ends to aide influid flow. The nonexpandable portion 623 of cannula 625 may be formedfrom conventional biocompatible polymer tubing and the like. Theexpandable portion 626 of cannula 625, on the other hand, may be formedfrom a mesh 631, such as a metal or polymer mesh, and an elastomercoating 633. The mesh predominantly defines the radial stiffness andbending characteristics of the cannula, while the elastomer coats themesh to form a continuous duct having a fluid-carrying capability.

Mesh 631 may be in the form of a hexagonal cell matrix, or may includecircumferential rings 692 and axial connectors 694, as shown in FIG. 16.The circumferential rings predominantly control the radialcharacteristics while the axial connectors affect axial stiffness andbending performance.

Mesh 631 may be formed from a flexible material, such as a polymer,metal, any shape memory material, or other material, and may include amachined metal cylinder with laser cut voids, a matrix of woven wires,or other configuration. Where mesh 631 is made from a memory metalalloy, such as nitinol, a constant diameter tube of the metal, having ametal thickness on the order of thousandths of an inch, for example, athickness in the range of 0.005-0.007 inch, may be cut using a laser soas to leave a mesh structure. The constant-diameter mesh may then beexpanded/contracted radially to the desired shape using a mandrel, andoptionally a clamping mechanism may be used to ensure the mesh conformsto the mandrel geometry. The material is “shape set” to thisconfiguration using, for example, heat treatment. The mandrel, and hencethe diameter profile of the expandable portion 626 of cannula 625,optionally can be customized to a particular patient. Alternatively,mesh 631 may be formed from a polymer. Other suitable materials for mesh631 include other metals (such as alloys, including other memory metalalloys), polymers, other shape memory materials, and the like.

Use of the laser-cutting and shape-setting steps enables complicatedgeometric patterns to be formed from the constant-diameter tube. Anexample cannula design may include a bell-mouth inlet (to minimizehydrodynamic losses), a hydrodynamic diffuser at the outlet (forpressure recovery from fluid velocity), a screen-like device at theinlet end (for avoidance of inlet flow obstructions), and additionalmaterial at the screen tip that serves as a dilator when the cannula iscontracted.

Once mesh 631 has been formed, a coating, such as elastomer coating 633,may be applied to the mesh inner surface, outer surface and/orinterstitially. The coating (which may be, for example, biocompatible,corrosion resistant and/or flow improving) may be formed by a solutioncasting method or by other techniques known in the art, includingforming the coating as a separate tube, fitting it over the mesh andheat shrinking it to produce a tight fit. An elastic polymer such asElastane™ or Biospan™ may be used for coating 633, as may otherpolyurethanes, or other polymers. Mesh 631 and coating 633 may provide aflexible, expandable portion 626 of cannula 625 that is a conduit forfluid flow. The expandable portion 626 of cannula 625 may be generallycylindrical with a flow inlet 642 at its distal end and a flow outlet644 at its proximal end.

The mesh 631 is radially expansible in a way which imparts a minimallength change (along the axial direction) during radialexpansion/contraction. The expandable portion 626 of cannula 625 mayradially contract or expand using stored potential energy, and thus ispreferably a self-expanding/self-contracting device.

The radial stiffness of the expandable portion 626 is controllable viathe mesh thickness and the geometric density of the cell structure,which can vary along the cannula length. Such variability is useful tomatch the cannula stiffness with the imposed hydrodynamic loading,enabling a nearly constant radial deflection of the tube when operatingas a flow duct (wherein the hydrodynamic pressure varies along thelength). This is important in the region of the impeller to provide aconstant operational tip gap.

Cannula bending stiffness is also a controllable parameter that may varyaxially. For example, where circumferential rings 692 and axialconnectors 694 are used to form mesh 631, the bending stiffness ispredominantly controlled by the number and placement of the axialconnectors, but also depends on the stiffness of the circumferentialrings and the stiffness of the elastomer coating 633. The relativeplacement of the circumferential rings largely affects the radialstability of the cannula during bending. For example, as shown in FIG.16, mesh 631 may have a substantial amount of interleaving of adjacentcircumferential rings. This configuration yields a very stable cannulawith respect to radial buckling caused by a bending deflection.Conversely, a mesh pattern with no interleaving yields a cannula that isprone to radial buckling during a bending deflection. Radial stiffnessmay be augmented via mesh thickness or mesh density. A dense meshexhibits greater radial stability than a less dense mesh.

FIG. 17 depicts cannula 625 in the expanded state. The expanded portion626 of cannula 625 includes a distal end 646 having an inlet 642 throughwhich blood enters the cannula, and a proximal end 648 having an outlet644 through which blood leaves the cannula. The portion between inlet642 and outlet 644 is the expandable portion 626 of cannula 625. Inlet642 may be provided with a plurality of inlet struts 652 which preventobstructions from entering the cannula. Similarly, outlet 644 may beprovided with a plurality of discharge struts 654 which act asstationary stator blades and remove swirl velocity from the dischargeflow of impeller 605. Inlet struts 652 and discharge struts 654 mayoccupy a short section of the cannula assembly (such as 1 cm) and may beflat linear elements arranged in a uniform circular disposition aboutthe central axis of the device or may be part of mesh 631.Alternatively, struts 654 may be formed with airfoil typecross-sections. Impeller 605 is located close to the proximal end 648,and a guide wire 680 extends through cannula 625 and through hub 610 ofimpeller 605. The blood flow through the expanded portion 626 of cannula625 is from right to left (as shown in FIG. 17) for an LVAD, bloodentering the device through the distal end 646 and leaving the devicethrough the proximal end 648.

An uncut region of the original tube retains the original tube diameterand may be used as the storage housing 660 in the form of a cup forretaining impeller 605 in the stored configuration. Storage housing 660may be referred to as a non-expanded portion of cannula 625 throughwhich blood does not flow. In this case, the expandable portion 626 ofcannula 625 is attached to storage housing 660 through the dischargestruts 654. Alternatively, the cannula mesh 631, discharge struts 654and storage housing 660 may be formed from a single tube formed from thesame. The cannula inlet struts 652 tube. Hence, the cannula can and alsobe various attached components can be manufactured from a single pieceof tube, for example from a nitinol tube using laser cutting, with amandrel used for shaping the mesh portion. Alternatively, variousportions of the cannula can be manufactured separately and attachedtogether using welding or other attachment techniques. Storage housing660 may have a flared end 649 which may be defined by the shape ofdischarge struts 654 to aid in moving impeller 605 back to its storedposition.

Impeller 605 may be held in storage housing 660 m the storedconfiguration and moved axially into expandable portion 626 fordeployment, such as by using drive shaft 630 to urge the impeller out ofthe storage housing. Impeller 605 then unfolds into the deployedconfiguration using the stored potential energy of blades 612 in thestored configuration. Alternatively, impeller 605 may be held in thestored configuration in the expandable portion 626 while contracted, andmay deploy automatically upon expansion of the expandable portion. Instill other embodiments, storage housing 660 itself may be expandable,allowing impeller 605 to expand to its deployed diameter without axialmovement.

In an example of cannula 625 described above, the expandable portion 626thereof was formed from a nitinol tube having an inner diameter of 2.62mm, an outer diameter of 3.02 mm, and a length of 150 mm. In theexpanded state, portion 626 had a nominal inner diameter of 6.6 mm inthe expanded section, and a nominal length of 133 mm. The expandableportion included 35 circumferential rings 692, four axial connectors 694per ring in a fully connected region (involving eight circumferentialrings), and one axial connector 694 per ring in a minimally connectedcircumferential rings). Each region (involving twenty-eightcircumferential ring 692 had four waves per ring, with a wave amplitudeof 5.05 mm (at cut diameter). The interleaved fraction of the rings was2.05/5=(where the interleaved fraction for fully interleaved is 1, theinterleaved parameter being the overlapped distance divided by the waveamplitude). Finally, the typical thickness of inlet struts 652 anddischarge struts 654 was 0.2 mm.

A rotatable drive shaft 630 provides rotational coupling between a motor(not shown), located outside of the patient, and the impeller 605. Driveshaft 630 may have a substantially rigid portion 632 at its distal endwhich is connected to impeller 605, and a substantially flexible portion634. The flexible portion 634 of the drive shaft may be housed within aflexible tube 638 which supports the flexible portion and maintains itsshape as it is driven rotationally. The proximal end of drive shaft 630may be connected to the motor for rotating the drive shaft and with itimpeller 605. Alternatively, drive shaft 630 may be omitted, and theelectric power may be provided through a proximal portion of theassembly to operate a pump motor and impeller 605.

Drive shaft 630 may have a diameter on the order of 1.1 to 2 mm, and maybe hollow to allow guide wire 680 to pass therethrough. The flexibleportion 634 of drive shaft 630 may be formed from a metal or polymerbraid which is easily bendable so as to achieve a bend radius on theorder of 1 cm. Commercially available flexible impeller drive shafts maybe used in blood pump 600, such as those formed from metal wireconstruction. However, heating problems due to friction between rotatingand non-rotating components may occur within any small radius bendsrequired for operation. A composite flexible shaft may be used to reducesuch heating problems. The heating problem also can be addressed byproviding lubricating or low-friction films on one or both of adjacentsurfaces with high relative rotational motion. In that regard, apreferred drive shaft 630 may be constructed from coiled stainlesssteel, with an optional polymer support tube. A particularly preferredpolymer is polytetrafluorethylene.

The rigid portion 632 of drive shaft 630 may be supported by one or morebearings 672 retained in a bearing housing 675. A saline solution may bedirected into bearing housing 675 through internal lumen 670, and thebearing unit end seal 674 may be dimensioned so that a of clean salinesolution is infused (approximately 1-2 cc/hr). This fluid very smallquantity into the patient flow helps clean impeller 605 and dampensdrive shaft vibrations. The fluid flow may also prevent blood fromentering bearing housing 675 and compromising its operation and life. Ifthe density of drive shaft 630 is approximately the same as that of thesaline solution or other introduced fluid, most of the vibration can bedamped. Drive shaft 630 may be formed from carbon or other fiber andpolymer composite which has a lower density than metal and more closelymatches the density of the saline solution. Other lower density draftshafts and/or higher density fluids may be used for vibration damping.The saline solution or other fluid may be introduced to bearing housing675 through openings 678 in hollow drive shaft 630.

FIGS. 23A and 23B show an embodiment of blood pump 600 having an axiallyslidable storage housing 660. As can be seen in these figures, bearinghousing 675 may have a reduced diameter portion 677 between its endshousing bearings 672. This reduced diameter portion thus defines alongitudinal space for sliding movement of an internal rib 662 definedby an indented annular channel 664 in storage housing 660. In thedeployed condition shown in FIG. 23A, storage housing 660 has been movedproximally by the maximum extent permitted by the engagement of internalrib 662 with a proximal shoulder of bearing housing 675, therebyrevealing blades 612 of impeller 605 for deployment. In the storedconfiguration shown in FIG. 238, on the other hand, storage housing 660has been moved distally to the maximum extent permitted by internal rib662 contacting a distal shoulder of bearing housing 675. In thisposition, the distal end of storage housing 660 surrounds blades 612 ofimpeller 605, retaining them in the stored configuration.

The internal lumen 670 of blood pump 600 receives guide wire 680.Together, lumen 670 and guide wire 680 assist in positioning blood pump600 within the patient. Guide wire 680 may have a two-part structure toassist in threading the guide wire through the bearing/seal assembly andthrough the hub 610 of impeller 605, since part of that threading may beaccomplished under factory controlled conditions, rather than at thetime of use. In one embodiment, guide wire 680 may have a J-tip 682which facilitates navigation of the tortuous arterial pathway from thefemoral insertion site to the cardiac left ventricle chamber. Guide wire680 may include an optional device on its proximal end to allow theattachment of a similar diameter extension of the guide wire, residingin the collapsed cannula 625. The lumen 670 in blood pump 600 may have arelatively large diameter relative to the diameter of guide wire 680.Guide wire 680 may have one or more additional distal end features suchas a spherical shape, or a valve plug 689 to plug a hole in impeller 605after withdrawal of the guide wire (see FIG. 22A).

Optionally, the guide wire channel extending through impeller 605 andthe bearing unit end seal 674 may have a valve action, as is known inthe art, sealing the guide wire passage after removal of guide wire 680.Guide wire 680 may leave a mechanical seal, not shown, upon removal, orthe material of impeller 605 may be designed so as to close the openinginto lumen 670 upon removal of the guide wire. This avoids excess salineinfusion into the patient.

Blood pump 600 may be inserted into the patient's body using asheathless insertion procedure. Such procedure may employ a retainersheath 700 having a distal portion 702 and a proximal portion 704, asshown in FIG. 15C. Distal portion 702 may be about 20 cm in length, andhave an inner diameter of about 9 fr (3.0 mm) and an outer diameter ofabout 10.5 fr (3.5 mm). The inner diameter of the distal portion allowsstorage of the collapsed cannula/impeller assembly. The proximal portion704 of retainer sheath may be about 1 meter in length with an outerdiameter of about 9 fr. This proximal portion 704 may serve as thehousing for the flexible portion 634 of drive shaft 630 and for thenon-expandable portion 623 of cannula 625.

A “pre-parked” integrated insertion sheath 800 may slide over theproximal portion 704 of retainer sheath 700. The outer diameter ofinsertion sheath 800 is preferably about the same as the outer diameterof the distal portion 702 of retainer sheath 700, in this example about10.5 fr. When the proximal end of the distal portion 702 of retainersheath 700 is pulled up against the distal end 802 of insertion sheath800, a smooth transition is evident, as shown in FIG. 25. Thus, thecombined retainer sheath 700 and insertion sheath 800 may be insertedinto the patient's femoral artery as a single entity. After theinsertion sheath is fully inserted into the femoral artery, the proximalportion 704 of retainer sheath 700 may be pushed into the patient,pushing the distal end of the retainer sheath and its contents into thepatient's left ventricle.

The most distal end 710 of retainer sheath 700 may have a series ofslots (not shown) that allow slight expansion of the retainer sheathdistal end during removal of cannula 625. As the expandable portion 626of cannula 625 must be collapsed during this process, the funnel shapecreated by these slots and the subsequent bending of the material of thesheath facilitates the collapse of the expandable portion of thecannula. Alternate means may be provided to facilitate the recollapse ofthe expandable portion 626 of cannula 625.

The drive motor rotates drive shaft 630 without rotating cannula 625 orretainer sheath 700. The operation of blood pump 600 is controlled andmonitored by a control unit (not shown) which displays status andcontrols various functions. Sensors, such as a pressure sensor and aflow rate sensor, may be affixed to various regions of the patientand/or blood pump 600.

The control unit preferably displays rpm of the drive motor, patientblood pressures, blood flow rate, information as to the location of theblood pump in the left ventricle, saline infusion and discharge rates,saline infusion temperature, etc. A filter may also be provided to showthe presence of debris or blood in the saline discharge stream. Theheart rate and blood flow rate are useful to be able to reduce patientdependency on the machine during recovery.

Detailed Description of the Deployment of the Heart Assist Device

Blood pump 600 may be percutaneously inserted through the femoral arteryand threaded toward the heart for use, for example, as a leftventricular assist device. Blood pump 600 may be inserted into a patientusing conventional cannula insertion methods. The impeller 605 of thedevice is then expandable in situ to enable an increased blood pumpingcapacity compared to conventional non-expandable devices. This caneliminate the requirement for surgical intervention.

Insertion may be accomplished using the Seldinger technique which iswell known in the art and used daily by surgeons and interventionalcardiologists. In such technique, an introducer needle (not show) isinserted into the femoral artery and used to introduce guide wire 680.Once guide wire 680 is in place, the needle is withdrawn. An optionalpredilator (not shown) can be used over guide wire 680 to open up thearteriotomy (an opening in the femoral artery) to a size appropriate forinsertion of blood pump 600.

A guide wire extension 685 (see FIG. 26) contained within the collapsedcannula 625 may be attached to the attachment device at the proximal endof guide wire 680. By fixing the guide wire extension at the proximalend of guide wire 680, the entire assembly is moved along the guide wirethrough the femoral artery opening previously created. During thisprocess, the attachment of guide wire 680 to the guide wire extension685 enters the body of the collapsed cannula assembly. To facilitateinsertion without an introducer sheath, the distal end of the device maybe provided with a tapered distal end dilator 688, shown in FIGS. 20 and22. This may be a compressed form of the inlet 642 of cannula 626 inwhich the inlet struts 652 fold down into a tapered, closedconfiguration similar in profile to a conventional dilator tip.

The blood pump 600, in its collapsed state, is then threaded over guidewire 680 and inserted into the artery. Once blood pump 600 ispositioned, guide wire 680 may be removed. A nose bearing or seal at thedistal end of impeller 605 can then seal the guide wire opening throughthe hub 610 of the impeller. This allows saline solution to be injectedinto impeller 605 for cooling and lubrication purposes, as well as toprevent blood from entering the lumen 670 of blood pump 600.

At the time of insertion, the expandable portion 626 of cannula 625 andimpeller 605 are collapsed and may be contained within retainer sheath700. As described above, the proximal end 704 of retainer sheath 700 mayinclude an optional second integrated insertion sheath 800 whichreplaces the function of the separate introducer sheath when the deviceis positioned in place. If there is no “introducer” sheath presentduring the insertion of the assist device into the femoral artery, theprocess is referred to as sheathless insertion. In FIG. 25, a smoothtransition from the diameter of insertion sheath 800 to the maximumdiameter of retainer sheath 700 is shown.

During the insertion of blood pump 600, the transition reg10n haseffectively zero length and zero change in outer diameter. Thetransition region is the region where the enlarged distal portion 702 ofretainer sheath 700 meets the end 802 of insertion sheath 800. After thecollapsed cannula assembly is inserted into the femoral artery, and theposition of insertion sheath 800 is fixed at the patient boundary, thetransition region will be located several centimeters past the femoralartery opening. At this point, the medical practitioner holds theexterior of insertion sheath 800 stationary and continues to push theretainer sheath assembly along guide wire 680, until the distal end ofthe collapsed cannula and retainer sheath reside at the distal end ofthe guide wire, within the left ventricle cavity 900.

A possible location of the device for LVAD use is shown in FIG. 26. Thenon-expandable portion 623 of cannula 625 extends from the descendingaorta 905 and into the femoral artery, from which it exits the hostbody. Guide wire 680 may be advanced into the left ventricle 900 usingan optional guiding catheter (not shown), and a fluoroscope may be usedto establish proper positioning of the assist device.

Once blood pump 600 is properly positioned, retainer sheath 700 may beretracted, by a dimension of about 15 cm for some embodiments, allowingthe expandable portion 626 of cannula 625 to expand to the deployedconfiguration.

The final step in deploying blood pump 600 involves pushing impeller 605from its stored position within storage housing 660 and positioning itat a specified location within the expanded portion 626 of cannula 625.This may be accomplished by applying a small force to drive shaft 630while holding retainer sheath 700 at a fixed location. Retainer sheath700, in turn, holds the expanded portion 626 of cannula 625 at apreviously fixed location. Once no longer restrained by storage housing660, impeller 605 expands to the deployed configuration due to theaction of stored strain energy.

The expandable portion 626 of cannula 625 may attain its deployedconfiguration through the action of stored strain energy (potentialenergy). This process reveals the cannula inlet 642, the expandedportion 626 of the cannula and the cannula outlet 644. Successfuloperation requires that the cannula inlet 642 reside in the leftventricle 900 of the heart and that the outlet 644 reside in the aorta.A fluid seal must exist where the cannula is proximate to the aorticvalve, and the surface smoothness of the cannula is preferably such thatclinically significant abrasion of the aortic valve is prevented. Also,distal end struts 652 form an inlet grid which prevents the inlet frombecoming blocked by soft tissues within the left ventricle.

Impeller 605 may be moved toward the distal end of cannula 625 whichcurves around through a valve into the left ventricle 900 of the heart,while the flexible portion 634 of drive shaft 630, coupled to impeller605, extends outside of the body of the patient and is rotated by adrive motor. The non-expanded portion 623 of cannula 625 similarlyextends through the femoral artery and outside of the patient. The inlet642 and struts 652 at the distal end of cannula portion 626 allowsubstantially unrestricted flow of blood into the device, where it isdriven by impeller 605 outside of the device through a discharge mesh orstruts 654 at the proximal end 648 of cannula portion 626.

Other methods of expanding impeller 605 may be used. One possiblealternative approach may include infusing a liquid or gas through ashaft to inflate impeller 605. Another approach may use rotationalforces to induce blades 612 to form a desired shape. The potentialenergy in the blades, particularly in the blade roots, may be used todeploy the blades into their unstrained position, and hydrodynamicforces may cause the blades to further deform into their operatingconfiguration.

The retainer sheath 700, previously retracted, serves to fix theposition of the entire assembly within the patient. Blood pump 600 isnow deployed and ready for connection to supporting equipment and use.

When the patient recovers and can be weaned from the necessity of usingblood pump intervention, impeller 605 may be pulled back into aninactive compressed configuration in storage housing 660 or in anon-expandable portion 623 of cannula 625, and the expandable portion626 of the cannula may be pulled into retainer sheath 700. Slots (notshown) or an outward flare 649 (see FIG. 9) may be provided at thedistal end of storage housing 660 to assist in retraction of cannula625. Retainer sheath 700 may then be pulled into proximity withinsertion sheath 800, and the insertion sheath, retainer sheath andcannula within it may be removed from the patient through the originalfemoral artery site. Subsequently, the wound in the patient may beclosed in a conventional fashion. The flow rate and pressure rise ofblood pumped by blood pump 600 is greater than current non-collapsibledevices of the same diameter, and the rate of blood damage (hemolysis)is maintained at a clinically acceptable low level. The use of thedevice as an RVAD is similar to that described above.

The expans10n feature of blood pump 600 is an advantage overnon-expandable prior art devices. If the device were non-expandable, themaximum cross-section would be limited to approximately 3 mm to allowfor percutaneous insertion. However, this cross-section is insufficientto achieve sufficient blood flow to maintain the health of the patient.

Other applications of the device according to the present inventioninclude providing additional blood flow to other organs, assisting theheart during operations, and the like.

The expandable portion 626 of cannula 625 may be expanded by any desiredmethod. In one approach, mesh 631 may expand in a radial direction whenthe expandable portion 626 is contracted along an axial direction, forexample using a mesh 631 having a hexagonal structure. In this approach,by applying tension to guide wire 680 through bull nose grommet 684,shown in FIG. 21A, the expandable portion 626 of cannula 625 can beshortened in the axial direction, providing radial expansion into theexpanded state. Where a shape-memory material is used for the expandableportion of the cannula, the cannula will achieve its expanded state asthe shape memory material reaches a predetermined temperature, such aswhen the cannula is inserted into a patient I s blood vessel. Bothimpeller 605 and cannula 625 in their stored configurations may be heldwithin retainer sheath 700, whereby both may be mechanically deployed ormay self-deploy when removed from the retainer sheath.

Cannula 625 may have at least two configurations, including a storedconfiguration and an expanded (deployed) configuration. When used aspart of a blood pump, cannula 625 in the deployed configuration may beabout 20-30 cm long with a diameter of about 6-7 mm. In the storedconfiguration, cannula 625 may have a diameter of about 3 mm, allowingnonsurgical insertion of blood pump 600 into a human subject through afemoral artery. The larger deployed diameter allows for higher fluidflow rates after insertion, and reduced friction pressure lossescompared with a non-surgically inserted blood pump having anon-expandable cannula.

An improved process for blood pumping within a living subject includesproviding an expandable impeller, inserting the impeller into a patientin a stored configuration (for example, with a diameter of between about3 mm and about 4 mm), positioning the impeller at a desired locationwithin a blood vessel of the patient, deploying the impeller (forexample, to a diameter of between about 6 mm and about 7 mm), andoperating the impeller in an operating configuration at a Reynoldsnumber of between about 1,000 and about 30,000, and preferably betweenabout 2,000 and about 10,000. Higher Reynolds number operation and moreefficient pump operation may be possible with higher rotation speeds,but may increase the destruction of structures within the pumped fluid,such as blood cells. The operating diameter of the impeller may be atleast about 50% greater than the stored diameter. In other examples, theoperating diameter may be at least about 100% greater than the storeddiameter. For animals, components may be scaled according to the size ofthe animal.

Novel configurations and material choice allow the improved device to becompressed for cannula insertion into a patient. Representative devicesinclude an expandable impeller, and a cannula that is at least in partexpandable, inside of which the impeller rotates. Both the expandableimpeller and the cannula have stored states that allow cannula insertioninto a vein or artery using non-surgical methods. After insertion andlocation of the device, the expandable impeller and cannula expand intodeployed states. The impeller can be driven through a flexible driveshaft from a drive motor external to the host, or using a motorproximate to the impeller, possibly integrated with a bearing system.

Other Pump Applications

Applications of the improved fluid pump designs described herein are notlimited to ventricular assist devices. The improved cannula and impellerdesigns are useful for any application where a stored configurationhaving reduced diameter is useful for locating the pump at a desiredlocation. For example, a fluid pump operating underground may beintroduced into a pipe, channel, or cavity through an opening of lesserdiameter, and operate at a diameter greater than that of the openingused. Applications of an impeller deploying within an expandable cannulainclude a collapsible fire hose with an integral booster pump, acollapsible propeller, a biomedical pump for a biological a fluid, andthe like.

In other examples, impellers may also be formed from metal sheets,plastic and non-resilient materials, for example in foldableconfigurations. Deployment may include the use of motors or othermechanical devices to unfold blades, automatic deployment induced bycentrifugal forces, and the like.

Although the invention herein has been described with referenceunderstood that to particular embodiments, these embodiments are merelyit is to be illustrative of the principles and applications of thepresent invention. It is therefore to be understood that numerousmodifications may be made to the illustrative embodiments and that otherarrangements may be devised without departing from the spirit and scopeof the present invention as defined by the appended claims.

What is claimed is:
 1. A catheter pump comprising: a cannula comprising:a non-expandable portion; an expandable portion extending between aproximal end and a distal end, wherein the expandable portion comprisesa mesh; a plurality of inlet struts extending from the expandableportion distal end; and a plurality of discharge struts extendingbetween the non-expandable portion and the expandable portion proximalend, wherein the plurality of inlet struts and the plurality ofdischarge struts comprise nitinol; and an impeller disposed in thecannula, the impeller sized and shaped to be inserted into a heart of apatient.
 2. The catheter pump of claim 1, wherein a proximal portion ofthe expandable portion is caused to collapse by advancement of aretainer sheath over the plurality of discharge struts.
 3. The catheterpump of claim 1, wherein the mesh comprises the plurality of inletstruts and the plurality of discharge struts.
 4. The catheter pump ofclaim 1, wherein the mesh, the plurality of inlet struts, the pluralityof discharge struts, and the non-expandable portion comprise acontinuous structure.
 5. The catheter pump of claim 1, wherein one ormore of the plurality of discharge struts are sized and shaped to permitan outward flow of blood from the expandable cannula.
 6. The catheterpump of claim 1, further comprising a fluid outlet to permit the flow offluid out of the apparatus, the fluid outlet at least partially definedby the plurality of discharge struts.
 7. The catheter pump of claim 1,wherein the plurality of inlet struts are configured to preventobstructions from entering the cannula.
 8. The catheter pump of claim 1,wherein the plurality of discharge struts are configured to act asstationary stator blades and remove swirl velocity from a discharge flowof the impeller.
 9. The catheter pump of claim 1, wherein the pluralityof discharge struts are flat linear elements.
 10. The catheter pump ofclaim 1, wherein the plurality of discharge struts have an airfoilcross-section.
 11. A cannula comprising: a non-expandable portion; anexpandable portion extending between a proximal end and a distal end,wherein the expandable portion comprises a mesh; a plurality of inletstruts extending from the expandable portion distal end; and a pluralityof discharge struts extending between the non-expandable portion and theexpandable portion proximal end, wherein the plurality of inlet strutsand the plurality of discharge struts comprise nitinol.
 12. The cannulaof claim 11, wherein a proximal portion of the expandable portion iscaused to collapse by advancement of a retainer sheath over theplurality of discharge struts.
 13. The cannula of claim 11, wherein themesh comprises the plurality of inlet struts and the plurality ofdischarge struts.
 14. The cannula of claim 11, wherein the mesh, theplurality of inlet struts, the plurality of discharge struts, and thenon-expandable portion comprise a continuous structure.
 15. The cannulaof claim 11, wherein one or more of the plurality of discharge strutsare sized and shaped to permit an outward flow of blood from theexpandable cannula.
 16. The cannula of claim 11, further comprising afluid outlet to permit the flow of fluid out of the apparatus, the fluidoutlet at least partially defined by the plurality of discharge struts.17. The cannula of claim 11, wherein the plurality of inlet struts areconfigured to prevent obstructions from entering the cannula.
 18. Thecannula of claim 11, wherein the plurality of discharge struts areconfigured to act as stationary stator blades and remove swirl velocityfrom a discharge flow of the impeller.
 19. The cannula of claim 11,wherein the plurality of discharge struts are flat linear elements. 20.The cannula of claim 11, wherein the plurality of discharge struts havean airfoil cross-section.